290 EDEMA 



NaCl solution, which has a pressure of over thirty atmospheres. 

 When we consider that in his studies on lung edema Welch 

 was able by ligation of the aorta to raise the blood pressure 

 less than ^_ atmosphere, we begin to appreciate how much more 

 powerful are the forces of osmotic pressure that are at work in 

 the body than is the blood pressure, even of the aorta itself. 



Loeb found that whenever oxidation is impaired in a tissue its 

 osmotic pressure rises, due to the accumulation of incompletely 

 oxidized metabolic products, particularly acids, and as a result the 

 muscle takes up water and becomes edematous. On this basis 

 we may explain the edema of venous stagnation as due to accu- 

 mulation of products of metabolism, partly because of impaired 

 oxidation, partly, perhaps, because of their slow removal in the 

 blood on account of the circulatory disturbance. The so-called 

 "neurotic" edemas may possibly be explained by local increase 

 in metabolic activity brought about by nervous stimuli, which 

 causes increased formation of substances raising osmotic pressure 

 in the stimulated tissues. In renal edema the retention of water 

 also seems to depend rather on osmotic pressure than on cir- 

 culatory disturbances or alterations in the vessel-walls, for it 

 has been shown that retention of chlorides, which the diseased 

 kidneys do not eliminate normally, is an important cause of the 

 dropsy. The chlorides accumulating in the tissues lead to an 

 increased osmotic pressure, which causes the abstraction of water 

 from the blood and its retention in the tissues. (The details of 

 this subject will be considered later.) Conversely, Meltzer and 

 Salant found that salt solution is absorbed from the peritoneal 

 cavity more rapidly in nephrectonized rabbits than in normal 

 rabbits, because metabolic products accumulate in the blood and 

 raise its osmotic pressure above normal. 



There are some difficulties, however, in applying the influence 

 of osmotic pressure as an explanation of all edemas. For 

 example, in edema of the lungs, as Meltzer points out, what is 

 the force that drives the fluid into the empty air-cells? Equally 

 difficult to explain as the result of osmotic disturbance is the 

 distribution of fluid that is seen in cardiac dropsy. The fluid 

 does not accumulate in the tissues where metabolism is greatest, 

 or where the most oxygen is used; but rather in the inactive 

 subcutaneous tissues and in the serous cavities. Possibly the 

 original trans udation does occur in the muscles and solid 

 viscera, and the fluid is then mechanically forced out of them 

 into the surrounding tissue-spaces, later settling according to 

 the laws of gravity or according to the distensibility of the 

 tissues. 



